There are several field devices to measure the power developed by a bicycle rider. In general, conventional apparatuses have measured only the tangential force (“FTAN”) component (or torque force), or some manifestation of it such as the torque generated in the bicycle driving sprocket due to the pedal force.
Conventional strain multipliers are not designed or intended to be accurate in the presence of off axis forces, or around axis torque, and as such, would not retain accuracy during normal usage on a bicycle. Some conventional strain measurement devices are adapted to measure the radial force component (“FRAD”), by placing sensors on the sides of the crank arm near sensors intended to measure FTAN. These devices do not deliver an optimal level of accuracy because the positioning of the sensors fails to adequately take into consideration the small magnitude of the forces acting upon the crank arm.
The maximum force that a powerful athlete can apply to the vertical crank is about three times body weight, or around 3500N. This generates the maximum value of the force FRAD. Since the cross sectional area of the crank is typically 350 mm2 then the maximum stress generated by the force in the bicycle crank arm will be 10 MPa. The bicycle crank is often constructed from high grade aluminium for which the Young's modulus is known to be approximately 70 GPa.
The formula relating stress and strain in the material is given by E(Young's modulus)=Stress/Strain. From this formula we get the strain in the material to be Strain=Stress/E=10×10^6/70×10^9=1/7000. Thus, the maximum signal generated at the strain gage in a direct application to measure strain will be: Signal=Strain×Bridge supply voltage×gage factor (of the strain gage) volts=(1/7000×3.5×2) volt=0.001 volt.
The analog system in a mixed signal microcomputer is particularly accurate if this maximum signal is amplified to a value similar to the working reference voltage. The signal needs to be a typical maximum, e.g., 3.5 volt, and therefore would be amplified by 3500 to be a signal suitable for processing. It is well known to those skilled in the art that it is difficult to achieve such a high gain and simultaneously achieve low power, low thermal drift and low noise in the amplifier system.